On the mail side, you'd have to some sort of routing (Ain't I helpful) with pop/imap servers that understand the routing or run some sort of shared storage (NFS, etc) and each box could server pop/imap clients (but some would be slow).
QmalLdap has the functionality built into it, Postfix and Sendmail will allow mapping with LDAP. Your pop/imap server will be the difficult ones to find.
SMTPAUTH is a nice solution, but still can be buggy depending on your MUA. Pop before Relay is still your best bet. In this distributed setup, you'll want to the pop-before-reply database shared among your servers.
OpenACS really doesn't have a mail server or routing controls built into it. So about the best we can offer right now is advice.
